1. Field of Invention
This invention is in the field of manufacture of semiconductor devices. More particularly, this invention relates to a method and system of fabricating underlying structures to improve planarization characteristics of layers while minimizing introduction of random and/or systematic noise by the underlying structures to the metrology signal.
2. Related Art
As feature sizes in semiconductor devices shrink, photolithographic equipment requires that layers of a wafer be very flat or planar so that small feature dimensions are accurately patterned. Chemical mechanical planarization (CMP) is widely used in various semiconductor processing operations to planarize layers of a wafer. The polishing process typically uses an abrasive slurry and a combination of mechanical and chemical actions to planarize the layer surface.
CMP performance is affected by the density of underlying structures in a wafer, resulting in two phenomena called xe2x80x9cdishingxe2x80x9d and xe2x80x9cerosionxe2x80x9d. FIG. 1A is an architectural diagram illustrating the xe2x80x9cdishingxe2x80x9d effects of the CMP process on a layer of a wafer. In a stacked wafer structure 13, the stack comprises a silicon layer or a substrate 11 and a dielectric layer or layers 9, a metal seed layer 7, and an interlayer dielectric (ILD) 5, typically silicon oxide. The metal layer 3, typically copper, aluminum, or tungsten, is planarized using a CMP process. Since the metal layer 3 is relatively softer than the ILD plus the chemical action of the CMP, a metal loss region 1 occurs, referred to as xe2x80x9cdishingxe2x80x9d.
FIG. 1B is an architectural diagram illustrating the xe2x80x9cerosionxe2x80x9d effects of the CMP process on a layer of a wafer. In a stacked wafer structure 20, an ILD layer 31 is fabricated above a substrate or previous layers (not shown). Another ILD layer 25 is fabricated above the etch stopper layer 29. The metal layer 23 is planarized using a CMP process where a different polishing rate applies to the dense region 24 compared to the region with small features 22. The difference in the polishing rate is most pronounced in the middle of the small feature region 22, resulting in a concave-profile loss region consisting of loss of ILD and metal. This phenomenon, referred to as xe2x80x9cerosionxe2x80x9d.
One of the solutions used to solve the local pattern density effect is to use fill or dummy shapes. U.S. Pat. No. 5,278,105 entitled xe2x80x9cSemiconductor Device with Dummy Features in Active Layersxe2x80x9d to Eden, et al., discusses the use of fill shapes for correcting problems related to local pattern density
FIG. 2 is an architectural diagram illustrating the use of an optical metrology system to acquire critical dimension (CD) data off target periodic structures. The optical metrology system 40 consists of a metrology beam source 41 projecting a beam 43 at the target periodic structure 53 of a wafer,43 mounted on a metrology platform 55. The metrology beam 43 is projected at an incidence angle xcex8 towards the target periodic structure 53. The reflected beam 49 is measured by a metrology beam receiver 51. The reflected beam data 57 is transmitted to a metrology profiler system 53. The metrology profiler system 53 compares the measured reflected beam data 57 against a library of calculated reflected beam data representing varying combinations of critical dimensions of the target periodic structure and resolution. The library instance best matching the measured reflected beam data 57 is selected. The profile and associated critical dimensions of the selected library instance correspond to the cross-sectional profile and critical dimensions of the features of the target periodic structure 53. A similar optical metrology system 40 is described in U.S. Pat. No. 5,739,909, entitled xe2x80x9cMeasurement and Control of Linewidths in Periodic Structures Using Spectroscopic Ellipsometry.xe2x80x9d, issued to Blayo, et al.
Two spectroscopic metrology techniques are used typically to measure target structures in a non-destructive manner: spectroscopic reflectometry and spectroscopic ellipsometry. In reflectometry, light intensities are measured. R=|r|2 is the relation between the reflectance R and the complex reflection coefficient r. In spectroscopic ellipsometry, the component waves of the incident light, which are linearly polarized with the electric field vibrating parallel (p or TM) or perpendicular (s or TE) to the plane of incidence, behave differently upon reflection. The component waves experience different amplitude attenuations and different absolute phase shifts upon reflection; hence, the state of polarization is changed. Ellipsometry refers to the measurement of the state of polarization before and after reflection for the purpose of studying the properties of the reflecting boundary. The measurement is usually expressed as tangent (xcexa8) and cosine (xcex94).
In spectroscopic metrology, the reflected optical signal is considered ideal when the underlying structure is unpatterned, i.e., like a unpatterned film or substrate. The presence of a pattern in the underlying structure may introduce random and/or systematic noise to the measured reflected optical signal. The source of random noise cannot be determined whereas systematic noise can possibly be determined and characterized. Significant random or systematic noise in the signal in turn skews the matching process of the measured reflected signal against the library of calculated reflected beam data, causing the match of the measured reflected signal to a different profile and/or different critical dimensions.
On the other hand, the underlying structure design may not introduce random and/or systematic noise to the measured reflected optical signal but the CMP characteristics of planarized layers might exceed ranges required by the application.
Thus, there is a need for identification of an underlying structure that achieves improved planarization characteristics of layers while minimizing introduction of random and/or systematic noise to the reflected metrology signal.
The present invention is a method and system for identifying an underlying structure that achieves improved planarization characteristics of layers while minimizing introduction of random and/or systematic noise to the reflected metrology signal.
One embodiment of the present invention is a method of optimizing the design of underlying structures in a wafer with one or more designs of pads of varying sizes and varying loading factors, comparing planarization characteristics of the underlying layers of the wafer to preset standard planarization characteristics, fabricating target structures in the target layer of the wafer, measuring a reflected metrology signal off a calibration test area to obtain a calibration metrology signal, the calibration test area having an unpatterned underlying structure, and selecting the design of pads that yield a reflected metrology signal closest to the calibration metrology signal and that meet preset standard planarization characteristics. Another embodiment is a method of designing underlying structures with one or more designs of random shapes of varying sizes and varying loading factors in underlying layers of a wafer.
Still another embodiment of the present invention is a method of optimizing the design of underlying structures with one or more periodic structures of varying line-to-space ratios in one or more underlying layers of a wafer, the periodicity of the underlying periodic structure being positioned at an angle relative to the direction of periodicity of the target periodic structure of the wafer. In one application, the angle relative to the direction of periodicity of the target periodic structure of the wafer is ninety degrees.
The present invention also includes a system for optimizing the selection of an underlying structure design that balances planarization and optical metrology objectives for a target structure comprising a wafer fabricator, a planarizer for planarizing a surface of an underlying layer of the wafer, a layer profiler for measuring the planarization characteristics of planarized layers of the wafer, an optical metrology device for processing the reflected signal from the target structure in the target layer of the wafer, and a selector for the selecting the design of underlying structure that yields a reflected metrology signal closest to the calibration metrology signal and that meets preset standard planarization characteristics.